In recent years, in connection with the use of finer devices accompanying the use of higher integration degree of semiconductor circuits such as DRAM, demand for quality of silicon single crystals produced by the Czochralski method (it is also abbreviated as “CZ method” hereinafter) from which substrates therefor are produced is becoming higher. In particular, since there are defects generated during the crystal growth, which are called grown-in defects such as FPD, LSTD and COP, and they degrade oxide dielectric breakdown voltage and device characteristics, reduction of density and size of these defects is considered important.
Prior to explanation of those defects, there will be given first general knowledge of factors determining densities of defects introduced into silicon single crystals, a vacancy type point defect called vacancy (also abbreviated as V hereinafter), and an interstitial type silicon point defect called interstitial silicon (interstitial-Si, also abbreviated as I hereinafter).
In a silicon single crystal, a V-region means a region containing many vacancies, i.e., depressions, holes and so forth generated due to shortage of silicon atoms, and an I-region means a region containing many dislocations and aggregations of excessive silicon atoms generated due to excessive amount of silicon atoms. And between the V-region and the I-region, there should be a neutral region (also abbreviated as N-region hereinafter) with no (or little) shortage or no (or little) surplus of the atoms. Further, it has become clear that the aforementioned grown-in defects (FPD, LSTD, COP etc.) should be generated strictly only with supersaturated V or I, and they would not be present as defects even though there is little unevenness of atoms so long as V or I is not saturated.
It is known that densities of these two kinds of point defects are determined by the relationship between the crystal pulling rate (growing rate) F, and the temperature gradient G in the vicinity of the solid-liquid interface in the crystal in the CZ method. It has also been confirmed that defects distributed in a ring shape called OSF (Oxidation Induced Stacking Fault) are present in the N-region between the V-region and the I-region. Since OSFs are generated in a shape of concentric circle observed in a wafer surface when the wafer is sliced from a single crystal, there is used a term of OSF ring.
Those defects generated during the crystal growth are classified as follows. For example, when the growth rate is relatively high, i.e., around 0.6 mm/min or higher, grown-in defects considered to be originated from voids, i.e., aggregations of vacancy-type point defects, such as FPD, LSTD and COP, are distributed at a high density over the entire cross-section of the crystal along the radial direction, and a region containing such defects is called V-rich region (region in which supersaturated vacancies form void defects). When the growth rate is 0.6 mm/min or lower, with the decrease of the growth rate, the aforementioned OSF ring is initially generated at the circumferential part of the crystal, and L/D (large dislocations, abbreviation of interstitial dislocation loops, which include LSEPD, LFPD and so forth), which are considered to be originated from dislocation loops, are present outside the ring at a low density. A region containing such defects is called I-rich region (region in which supersaturated interstitial silicons form dislocation loop defects). When the growth rate is further lowered to around 0.4 mm/min or lower, the OSF ring shrinks and disappears at the center of wafer, and thus the entire plane becomes the I-rich region.
Recently, there has been discovered a region called N-region between the V-rich region and the I-rich region, and outside the OSF ring, in which neither of the vacancy-originated FPD, LSTD and COP, the dislocation loop-originated LSEPD and LFPD and OSF are present. This region exists outside the OSF ring, and shows substantially no oxygen precipitation when it is subjected to a heat treatment for oxygen precipitation and examined by X-ray analysis or the like as for the precipitation contrast. This region is present at rather I-rich side, in which the defects are not so rich as to form LSEPD and LFPD.
Presence of the N-region was also confirmed inside the OSF ring, in which neither of vacancy-originated void type defects, dislocation loop-originated defects and OSFs were present.
Because these N-regions are formed obliquely with respect to the growing axis when the growth rate is lowered in a conventional growing method, it exists as only a part of the wafer plane.
As for this N-region, according to the Voronkov's theory (V. V. Voronkov, Journal of Crystal Growth, 59 (1982) 625-643), it was proposed that a parameter of F/G, which is a ratio of the pulling rate (F) and the crystal solid-liquid interface temperature gradient (G) along the growing axis, determined the total density of the point defects. In view of this, because the pulling rate should be constant in a plane, for example, a crystal having a V-rich region at the center, I-rich region at the periphery, and N-region between them is inevitably obtained at a certain pulling rate due to distribution of G in the plane.
Therefore, improvement of such distribution of G has recently been attempted, and it has become possible to produce a crystal having the N-region spreading over an entire transverse plane of the crystal, which region could previously exist only obliquely, for example, at a certain pulling rate when the crystal is pulled with a gradually decreasing pulling rate F. Further, such an N-region spreading over an entire transverse plane can be made larger to some extent along the longitudinal direction of the crystal by pulling the crystal at a pulling rate maintained at the value at which the N-region transversely spreads. Furthermore, it has also become possible to make the N-region spreading over the entire transverse plane somewhat larger along the growing direction by controlling the pulling rate considering the variation of G with the crystal growth to compensate it, so that the F/G should strictly be maintained constant.
That is, by pulling a CZ crystal with controlling F/G so that it should have the N-region for the entire plane, it has become possible to reduce the void type defects and dislocation clusters. However, there remains a problem that a margin for the control width of F/G is very narrow.
On the other hand, it is conventionally known that a silicon single crystal doped with nitrogen can reduce the defects in FZ silicon, and this method is also applied to the CZ method by utilizing the unique oxygen precipitation characteristics and so forth of nitrogen.
Therefore, in Japanese Patent Application No. 11-022919, the inventors of the present invention proposed production conditions for improving yield and productivity of wafers having the N-region for the entire plane utilizing the fact that the N-region is enlarged by doping nitrogen.
However, although the N-region containing no void type defects and no dislocation clusters, which are observed in the I-rich region, is surely enlarged to a significant degree with nitrogen doping, most of it consists of an N-region containing an OSF region, and enlargement degree of N-region containing no OSF region, which is practically usable as defect-free wafers, is relatively small.
Further, there is also a problem that the OSF region obtained with nitrogen doping shows an OSF nucleus density higher by several times compared with an OSF region obtained without nitrogen doping, and dislocation loops generated due to such OSF nuclei are present and adversely affect devices.